PURPOSE: To diagnose electrical fault such as disconnection of harness accurately by providing a check signal to a first drive signal fed from a valve opening setting means then inverting the signal through a drive means and producing a valve drive condition judging signal.

CONSTITUTION: A valve control amount setting means 20 outputs a first drive signal based on a basic control amount of a valve and a feedback correction amount, then the first drive signal is inverted through a drive means 21 into a second drive signal which is employed for control of an ISC valve 3. Output signal from the drive means 21 is fed to a valve drive condition judging means 22 where a valve drive condition judging signal is detected and compared with a check signal fed from a clock oscillating means 25. If there is an electrical fault such as disconnection of harness in the drive means 21, the valve drive condition judging signal is not detected through the valve drive condition judging means 22, but it is judged that there is an abnormality in the output from the drive means 21 and a fault signal is outputted. Then correction amount of a feedback correction amount setting means 19 is cleared and feedback control is stopped.